A. Field of the Invention
This invention relates to dual chamber pacemakers, and more particularly to a mode of operation which automatically determines whether an intrinsic atrial event occurring prematurely is physiological and pathological, and adapts its operation accordingly.
B. Background of the Invention
Conventional pacemakers which employ the automatic mode switching in upper rate response may induce pacemaker mediated tachycardia after an atrial arrhythmia has been terminated, when switching from the VVI or VVIR mode to the DDD or DDDR mode. During the first cardiac cycle after the mode switch, the origin of the first A-sense is unknown. It Could be either of atrial origin, i.e., a P-wave, or a retrograde P-wave caused by the previous V-pace. Also, with a Wenckebach response the AV delay is prolonged and thus the atrium has a longer period in which to repolarize and thereby provide a retrograde pathway for conduction, which may permit pacemaker mediated tachycardia. In U.S. patent application Ser. No. 226,654, filed Apr. 17, 1994, now U.S. Pat. No. 5,441,523 by Tibor Nappholz, incorporated herein by reference, hereinafter the Nappholz application a forced synchrony function is provided which inserts an A-pace if it is required, and thereby the retrograde pathway is made refractory so that a pacemaker mediated tachycardia cannot be induced by retrograde conduction.
However, another problem with existing pacemakers is that with the continued variability of the patient's physiology even the forced synchrony approach disclosed in the Nappholz application may become ineffective. In addition there is the ever present possibility of extraneous noise precipitating pacemaker mediated tachycardia (PMT). More specifically, during the initialization of a prior art dual chamber system, the physician must use a compromise in selecting the PVARP (Post Ventricular Atrial Refractory Protection) interval. On one hand, the physician must allow a sufficiently fast synchronized atrial rate tracking. On the other hand the patient's heart must be protected from pacer mediated tachycardia. In the Nappholz application, this is accomplished by adjusting the PVARP interval in accordance with the metabolic indicated rate. This approach provides effective protection at low metabolic rates however, the protection is lessened at higher rates. Another disadvantage of this approach is that, in younger patients the onset of normal sinus rhythm could be very fast and could exceed the shortening effect of the metabolic rate on the PVARP interval, resulting in loss of synchrony and the possible induction of PMT.